Pitfalls in measuring R waves in pacemaker-dependent patients.

Determination of adequate R wave sensing is an important step in pacemaker electrode implantation and pacemaker replacement operations. In patients who are completely pacemaker dependent, these operations are usually performed with a functional temporary pacemaker in place throughout the testing period. In such patients, there is the special problem of determining whether the test system is sensing the temporary pacemaker spike rather than the resultant QRS voltage. Patients were studied and a laboratory model was created to evaluate the response characteristics of a standard R wave test device. Patients showed two general types of curves as R waves were measured at various output voltages of the temporary pacemaker. Type A responses showed direct correlation between output voltage and measured R waves. Type B responses showed an initial high plateau of R waves followed by an abrupt fall below acceptable values, and then a direct correlation between R waves and output voltage. Laboratory testing revealed that the temporary pacemaker spike, and not the R wave, was being "sensed" in Type A curves, and that the R wave was sensed only in the initial plateau of the Type B curves. This pitfall can lead to acceptance of an unsatisfactory electode position in some patients and to futile electrode repositioning in others.